Convert char array to int

This is a discussion on Convert char array to int within the C Programming forums, part of the General Programming Boards category; Hey guys, I'm having a problem with converting characters from a char array to int variables. I've searched over the ...

  1. #1
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    Convert char array to int

    Hey guys, I'm having a problem with converting characters from a char array to int variables. I've searched over the forums but haven't been able to find an answer. What I'm trying to do is write a program that adds two fractions, where the fractions will be entered in by the user as 1/8 or 12/10 for example.

    The code below works fine for single digit fractions (eg: 1/8) but I cant get it to work for double digit ones.

    Here's what I have so far:

    Code:
     printf("Enter fraction #1: ");
       fgets(fractionOne, FR_MAX_INPUT+1, stdin);
    
       /* test to makesure that we have ALL of the line */
       if(fractionOne[strlen(fractionOne) - 1] != '\n') {
          /* the newline must be in the buffer */
          readRestOfLine();
       } else { /* get rid of the new line from the input */
          fractionOne[strlen(fractionOne) - 1] = '\0';
       }
    
       origFracOneNumerator = atoi (fractionOne);
       origFracOneDenominator = atoi (fractionOne+2);
    Now I thought it would be simply a matter of changing the line
    Code:
    origFracOneDenominator = atoi (fractionOne+2);
    to
    Code:
    origFracOneDenominator = atoi (fractionOne+3);
    But when I do this it tells me the value of origFracOneDenominator is 0 and not the second half of the fraction.

    Any ideas?

    Thanks in advance!

  2. #2
    Frequently Quite Prolix dwks's Avatar
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    Your job would be a lot easier if you used sscanf() or even strtol() instead of atoi(). And I'd recommend switching functions anyway, because atoi() doesn't do any error checking.

    strtol() can set a pointer to the character beyond the number it processed, which you can then increment until you pass a '/', then you can call strtol() again. Something like
    Code:
    char f[] = "12/34";
    char *p;
    long x, y;
    x = strtol(f, &p, 0);
    while(isspace(*p || *p == '/')) p ++;
    y = strtol(p, NULL, 0);
    With sscanf(), you can use
    Code:
    char f[] = "12/34";
    int x, y;
    if(sscanf(f, "%d/%d", &x, &y) != 2) { /* error */ }
    For more information, see
    http://www.cplusplus.com/reference/c...ib/strtol.html
    http://www.cplusplus.com/reference/c...io/sscanf.html
    dwk

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  3. #3
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    Thanks for that. I've only been programming C for about 8 weeks so I'm still a little unsure on what functions are best to use and when.

    Will give strtol a try and let u know how it goes.

    Thanks again.

  4. #4
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    Hmm, that seems to be doing the same thing. When I type in 12/13 it tells me x is 12 and y is 0.

    Am I missing something here?

  5. #5
    and the hat of wrongness Salem's Avatar
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    Post your latest code.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  6. #6
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    Code:
    char fractionOne[FR_MAX_INPUT+1];
       char fractionTwo[FR_MAX_INPUT+1];
       char *p;
       long x, y;
    
       printf("Fractions\n");
       printf("------------------\n");
    
       printf("Enter fraction #1: ");
       fgets(fractionOne, FR_MAX_INPUT+1, stdin);
    
       /* test to makesure that we have ALL of the line */
       if(fractionOne[strlen(fractionOne) - 1] != '\n') {
          /* the newline must be in the buffer */
          readRestOfLine();
       } else { /* get rid of the new line from the input */
          fractionOne[strlen(fractionOne) - 1] = '\0';
       }
    
       x = strtol(fractionOne, &p, 0);
       while(isspace(*p || *p == '/')) p ++;
       y = strtol(p, NULL, 0);
    
       printf("x is %d\n: ", x);
       printf("y is %d\n: ", y);

  7. #7
    CSharpener vart's Avatar
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    isspace(*p || *p == '/')
    shouldn't it be
    Code:
    isspace(*p) || *p == '/'
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  8. #8
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    Nah, I thought that at first but it still produces the same output.

  9. #9
    Woof, woof! zacs7's Avatar
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    There won't necessarily be a newline on the end of text entered into stdin, the FAQ demonstrates a good method on determining if there is - and to remove it if it's there.

  10. #10
    and the hat of wrongness Salem's Avatar
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    Works for me - note the change to the printf's at the end.
    Code:
    #include <stdio.h>
    #include <string.h>
    #include <stdlib.h>
    #include <ctype.h>
    
    #define FR_MAX_INPUT 100
    
    int main ( void ) {
       char fractionOne[FR_MAX_INPUT+1];
       char *p;
       long x, y;
    
       printf("Fractions\n");
       printf("------------------\n");
    
       printf("Enter fraction #1: ");
       fgets(fractionOne, FR_MAX_INPUT+1, stdin);
    
       /* test to makesure that we have ALL of the line */
       if(fractionOne[strlen(fractionOne) - 1] != '\n') {
          /* the newline must be in the buffer */
          /*readRestOfLine();*/
       } else { /* get rid of the new line from the input */
          fractionOne[strlen(fractionOne) - 1] = '\0';
       }
    
       x = strtol(fractionOne, &p, 0);
       while( isspace(*p) || *p == '/' ) p ++;
       y = strtol(p, NULL, 0);
    
       printf("x is %ld\n", x);
       printf("y is %ld\n", y);
       return 0;
    }
    
    $ ./a.exe
    Fractions
    ------------------
    Enter fraction #1: 12/34
    x is 12
    y is 34
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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